1. Field of the Invention
The present invention relates to a suck back valve which makes it possible, for example, to avoid liquid drip from a liquid supply port by sucking a predetermined amount of a fluid flowing through a fluid passage in accordance with a displacement action of a diaphragm.
2. Description of the Related Art
The suck back valve has been hitherto used, for example, in the production step for semiconductor wafers. The suck back valve has a function to prevent so-called liquid drip, i.e., a phenomenon in which a minute amount of coating liquid drips toward the semiconductor wafer from a supply port when the supply of the coating liquid to the semiconductor wafer is stopped.
A suck back valve concerning the conventional technique is shown in FIG. 5 (see, for example, Japanese Utility Model Publication No. 8-10399).
The suck back valve 1 includes a main valve body 5 formed with a fluid passage 4 for making communication between a fluid-introducing port 2 and a fluid-discharging port 3, and a bonnet 6 coupled to an upper portion of the main valve body 5. A diaphragm 7, which comprises a thick-walled section 7a and a thin-walled section 7b, is provided at a central portion of the fluid passage 4. The suck back valve 1 is formed such that the entire surface of the thick-walled section 7a and the thin-walled section 7b of the diaphragm 7 faces the fluid passage. The bonnet 6 is formed with a pressurized fluid supply port 8 which is connected to an unillustrated pressurized fluid supply source and to which a pressurized fluid is supplied in accordance with the changeover action of a directional control valve.
A piston 9 is fitted to the diaphragm 7. A v-packing 10, which is slidable on an inner wall surface of the main valve body 5 and which functions as a seal, is installed to the piston 9. A spring 11, which always presses the piston 9 upwardly, is provided in the main valve body 5. Reference numeral 12 indicates a screw member which abuts against the piston 9 to adjust the displacement amount of the piston 9 so that the flow amount of the fluid sucked by the diaphragm 7 is adjusted.
The outline of the operation of the suck back valve 1 will be explained. In an ordinary state in which the fluid is supplied from the fluid-introducing port 2 to the fluid-discharging port 3, the piston 9 and the diaphragm 7 are displaced downwardly in an integrated manner in accordance with the action of the pressurized fluid supplied from the pressurized fluid supply port 8. The diaphragm 7, which is coupled to the piston 9, protrudes over the fluid passage 4 as shown by two-dot chain lines in FIG. 5.
When the flow of the fluid through the fluid passage 4 is stopped, the piston 9 and the diaphragm 7 are raised in an integrated manner in accordance with the action of the repulsive force exerted by the spring 11, by stopping the supply of the pressurized fluid from the pressurized fluid supply port 8. A predetermined amount of the fluid, which remains in the fluid passage 4, is sucked in accordance with the action of the negative pressure produced by the diaphragm 7. Thus, the liquid drip, which would be otherwise caused at the fluid supply port, is prevented.
However, in the case of the suck back valve 1 concerning the conventional technique described above, the entire surface of the thick-walled section 7a and the thin-walled section 7b of the diaphragm 7 serves as a pressure-receiving surface for the fluid. Therefore, there is a likelihood that the diaphragm 7 is deformed due to the use over a long period of time, or any operation failure occurs due to occurrence of cracks or the like. As a result, an inconvenience arises in that the durability of the diaphragm 7 is deteriorated.
Further, the suck back valve 1 concerning the conventional technique involves the following inconvenience. That is, upon displacement of the piston 9, dispersion occurs in response speed when the state is changed from the fluid supply state to the fluid stop state or from the fluid stop state to the fluid supply state, due to the sliding resistance of the v-packing 10 which makes sliding displacement on the inner wall surface of the main valve body 5.
Moreover, the suck back valve 1 concerning the conventional technique involves the following inconvenience. That is, the screw member 12, which is screwed into the screw hole of the bonnet 6, is loosened, and the flow amount-setting position, at which the displacement of the piston 9 is restricted, is changed along with passage of time.